Consolidation and densification methods for fibrous monolith processing

ABSTRACT

Methods for consolidation and densification of multi-phase composite structures are provided. These methods allow for more efficient and less expensive consolidation and densification of two- and three-dimensional multi-phase components having more complex geometries.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on, and claims the benefit of,co-pending U.S. Provisional Application Serial No. 60/251,132, filed onDec. 4, 2000, and entitled “Alternative Consolidation And DensificationMethods For Fibrous Monolith Processing.”

[0002] This invention was made with U.S. Government support under grantNumber DE-FC02-96CH10861 awarded by the Department of Energy, and undergrant Number NAS8-00081 awarded by the National Aeronautics and SpaceAdministration. Accordingly, the Government may have certain rights inthe invention described herein.

FIELD OF THE INVENTION

[0003] The present invention relates to processes for consolidation anddensification of multiple-phase composite materials, including fibrousmonolith composites.

BACKGROUND OF INVENTION

[0004] The process of fabricating high strength materials from powderssuch as ceramic and metal powders generally involves preparing “green”materials that include the powder and a thermoplastic binder of variablecomposition. As part of the fabrication process, the binder typically isremoved from the material in a binder burnout step and the powderconsolidated and densified in order to obtain a final structure havingthe desired properties, including strength and hardness. Methods ofconsolidation and densification include sintering processes such as,uniaxial hot pressing, hot isostatic pressing, overpressure sinteringand atmospheric (pressureless) sintering.

[0005] Sintering processes, are critical in the fabrication of materialsfrom ceramic and metal powders. Equipment used in pressure sinteringprocesses including hot isostatic pressing (HIP) and uniaxial hotpressing must be designed to accommodate the high temperatures and highpressures associated with these sintering methods. Purchase, operationand maintenance costs for the HIP and uniaxial hot press equipment maybe high as a result of the need to incorporate vessels capable ofwithstanding high pressures or hydraulic controlled rams into theirrespective designs. There are also additional costs in addressing safetyrequirements and designs for the safe and reliable operation of highpressure equipment. Additionally, the capacity of HIP and uniaxial hotpress equipment is limited by these requirements. Thus, productionvolume capabilities are reduced, which further increases productioncosts. Furthermore, pressing is generally limited and cannot be usedeffectively with three-dimensional structures having more complexgeometries.

[0006] Pressureless sintering furnaces generally are less expensive topurchase, operate and maintain as compared to equipment for pressuresintering. They also provide larger production volume capabilities andlower overall production costs. However, an important disadvantageassociated with pressureless sintering is the potential inability toachieve effective sintering of a material in the absence of pressure.

[0007] Fibrous monoliths (FMs) are a unique class of structuralceramics. FMs are monolithic ceramics that are manufactured by powderprocessing techniques using inexpensive raw materials. Methods ofpreparing FM filaments are known. U.S. Pat. No. 5,645,781 describesmethods of preparing FM composites by extrusion of filaments havingcontrolled texture. As a result of the combination of relatively lowcosts of manufacture and benefits of enhanced materials performance, FMshave been used in a wider range of applications than heretofore typicalfor ceramics. Fibrous monoliths typically have been formed to variousfibrous textures. For example, FM filaments have been woven into thin,planar structures. Alternatively, the filaments have been formed intothree-dimensional structures having complex geometries.

[0008] Generally, the macroarchitecture of an FM composite includes aplurality of filaments each including a primary phase in the form ofelongated polycrystalline cells surrounded by at least a thin secondaryphase in the form of a cell boundary. The material selected for the cellphase differs from the material selected for the cell boundary phase intype and/or composition. Thus, the various materials comprising a FMcomposite each have different material properties.

[0009] This “multi-phase” nature of FM composites, along with thepossibility that the composites are formed into complex structures, canincrease the difficulties encountered when attempting to sinter suchcomposites. Significantly, when two or more materials are used and areto be maintained essentially separate from each other in a compositecomponent, the ability to effectively sinter the FM composite componentcan be severely limited or even prevented. Because the materialproperties of the two phases differ, the range of physical and chemicalconditions that lead to effective sintering of the composite can berestricted. The difficulty in identifying an effective sintering regimeincreases further as additional materials are included in the composite.Moreover, the potential for unfavorable interactions between materialsthat can limit sinterability increases as additional materials are addedto the composite.

[0010] There remains a need for more efficient, cost-effective sinteringprocesses that can be utilized during fabrication of fibrous monolithcomposite structures, particularly those having complex geometries.

SUMMARY OF THE INVENTION

[0011] The present invention overcomes the problems encountered inconventional methods by providing efficient, cost-effective processesfor consolidation and densification of composites formed of more thenone composition. More specifically, the present invention providesmethods of pressureless sintering that are effective for sinteringfibrous monolith composite structures, including those having complexgeometries. Pressureless sintering of FM composites provides for theconsolidation and densification of two- and three-dimensional componentsin less time and at a lower cost as compared to other sinteringprocesses. Additionally, FM composites with geometries too complicatedto be processed by uniaxial hot press techniques can be sintered inaccordance with the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention relates to methods of consolidating anddensifying ceramic composite components by pressureless sintering.Components that can be consolidated and densified in accordance with theinvention include those formed of composites that have two or morematerials present in essentially separate phases. Such compositesinclude fibrous monolith (FM) composites, which are made up of aplurality of filaments having a core phase that is surrounded by a shellphase.

[0013] In a pressureless sintering process, composites are heated tohigh temperatures without high pressure in a large volume, hightemperature furnace. In comparison to various pressure sinteringprocesses, pressureless sintering significantly lowers the overallproduction cost of FM composites, in part due to lower equipmentpurchase, operation and maintenance costs. Pressureless sintering alsoprovides large production volume capabilities, so that mass productionof FM components is possible. The processes of the present inventionthus provide increased effectiveness and efficiencies in the overallfabrication of FM composite components.

[0014] As used herein, “fibrous monolithic composite” and “fibrousmonolith” are intended to mean a ceramic composite material thatincludes a plurality of monolithic fibers, or filaments, each having atleast a cell phase surrounded by a boundary phase but may include morethan one core and/or shall phase. Fibrous monoliths exhibit thecharacteristic of non-brittle fracture, such that they provide fornon-catastrophic failure.

[0015] As used herein, “cell phase” is intended to mean a centrallylocated primary material of the monolithic fiber that is dense,relatively hard and/or strong. The cell phase extends axially throughthe length of the fiber, and, when the fiber is viewed in cross-section,the cell phase forms the core of the fiber. The “cell phase” also may bereferred to as a “cell” or “core”.

[0016] As used herein, “boundary phase” is intended to mean a moreductile and/or weaker material that surrounds the cell phase of amonolithic fiber in a relatively thin layer. The boundary phase isdisposed between the various individual cell phases, forming a separatelayer between the cell phase and surrounding cell phases when aplurality of fibers are formed in a fibrous monolithic composite. The“boundary phase” also may be referred to as a “shell,” “cell boundary,”or “boundary”.

[0017] Fibrous monoliths (“FMs”) are a unique class of structuralceramics that have mechanical properties similar to continuous fiberreinforced ceramic composites (CFCCs). Such properties includerelatively high fracture energies, damage tolerance, and gracefulfailures. In contrast to CFCCs, FMs can be produced at a significantlylower cost. FMs, which are monolithic ceramics, generally aremanufactured by powder processing techniques using inexpensive rawmaterials. As a result of the high performance characteristics of FMsand the low costs associated with manufacture of FMs, FMs are used in awider range of applications than heretofore typical for ceramiccomposites. Thus, FMs are used to form structures having a great varietyof shapes and sizes ranging from rather simple essentiallytwo-dimensional structures to very complex three-dimensional structures.

[0018] The macroarchitecture of an FM composite generally includesmultiple filaments each comprising at least two distinct materials—aprimary phase in the form of elongated polycrystalline cells separatedby a thin secondary phase in the form of cell boundaries. The primary orcell phase typically consists of a structural material of a metal, metalalloy, carbide, nitride, boride, oxide, phosphate or silicide andcombination thereof. The cells are individually surrounded and separatedby cell boundaries of a tailored secondary phase. Powders that may beused in the secondary phase include compounds to create weak interfacessuch as fluoromica, and lanthanum phosphate; compounds to createporosity in a layer which function to create a weak interface; graphitepowders and graphite-containing powder mixtures; and hexagonal boronnitride powder and boron nitride-containing powder mixtures. If ametallic debond phase is desired, reducible oxides of metals may beused, e.g., nickel and iron oxides, or powders of metals, e.g., nickel,iron, cobalt, tungsten, aluminum, niobium, silver, rhenium, chromium, ortheir alloys.

[0019] Advantageously, powders which may be used in the cell and/orboundary phase composition to provide the green matrix filament includediamond, graphite, ceramic oxides, ceramic carbides, ceramic nitrides,ceramic borides, ceramic silicides, metals, and intermetallics.Preferred powders for use in that composition include aluminum oxides,barium oxides, beryllium oxides, calcium oxides, cobalt oxides, chromiumoxides, dysprosium oxides and other rare earth oxides, hafnium oxides,lanthanum oxides, magnesium oxides, manganese oxides, niobium oxides,nickel oxides, tin oxides, aluminum phosphate, yttrium phosphate, leadoxides, lead titanate, lead zirconate, silicon oxides and silicates,thorium oxides, titanium oxides and titanates, uranium oxides, yttriumoxides, yttrium aluminate, zirconium oxides and their alloys; boroncarbides, iron carbides, hafnium carbides, molybdenum carbides, siliconcarbides, tantalum carbides, titanium carbides, uranium carbides,tungsten carbides, zirconium carbides; aluminum nitrides, cubic boronnitrides, hexagonal boron nitrides, hafnium nitride, silicon nitrides,titanium nitrides, uranium nitrides, yttrium nitrides, zirconiumnitrides; aluminum boride, hafnium boride, molybdenum boride, titaniumboride, zirconium boride; molybdenum disilicide; lithium and otheralkali metals and their alloys; magnesium and other alkali earth metalsand their alloys; titanium, iron, nickel, chromium, cobalt, molybdenum,tungsten, hafnium, rhenium, rhodium, niobium, tantalum, iridium,platinum, zirconium, palladium and other transition metals and theiralloys; cerium, ytterbium and other rare earth metals and their alloys;aluminum; carbon; lead; tin; and silicon.

[0020] Compositions comprising the cell phase differ from thosecomprising the boundary phase in order to provide the benefits generallyassociated with FMs. For example, the compositions may includeformulations of different compounds (e.g., HfC for the cell phase andWRe for the boundary phase or WC-Co and W—Ni—Fe) or formulations of thesame compounds with differing component amounts (e.g., WC-3% Co for thecell phase and WC-6% Co for the boundary phase) so long as the overallproperties of the compositions are not the same. For example, thecompositions can be selected so that no excessively strong bondingoccurs between the two phases in order to limit crack deflection.

[0021] The cell boundary phase may be selected to create pressure zones,microcrack zones, ductile-phasezones, or weak debond-type interfaces inorder to increase the toughness of the composite. For example,low-shear-strength materials such as graphite and hexagonal boronnitride make excellent week debond-type cell boundaries and are presentin Si₃N₄/BN and SiC/Graphite FM composites. The weak BN and graphiteinterfaces deflect cracks and determine thereby preventing brittlefailure of these composites and increasing their fracture toughness. Asa result, FM structures exhibit fracture behavior similar to CFCCs, suchas C/C and SiC/SiC composites, including the ability to fail in anon-catastrophic manner.

[0022] Fibrous monolith composites are fabricated using commerciallyavailable ceramic and metal powders using a process for convertingordinary ceramic powder into a “green” fiber that include the powder, athermoplastic polymer binder and other processing aids. Various methodsof preparing fibrous monolithic filaments are known in the art,including the methods disclosed in U.S. Pat. No. 5,645,781, which isincorporated by reference herein in its entirety. Generally, the fibrousmonolithic filaments that form the composite structures are prepared byfirst separately blending powders, polymer binders and possibly one ormore processing aids as the starting materials for the different phasesof the filaments. The materials of the cell and boundary are selected toprovide the final structures with predetermined properties. The startingmaterials are selected from a thermodynamically compatible set ofmaterials available as sinterable powders.

[0023] The fiber is compacted into the “green” state to create thefabric of elongated polycrystalline cells that resemble a fiber aftersintering or hot pressing. Once the green composite fiber is fabricatedit can be formed using any method known to those skilled in the art intothe shape of the desired component having, for example, conventionalcomposite architecture (e.g., uniaxial lay-up, biaxial lay-up, wovenfabric, etc.).

[0024] In final, finishing processes, the thermoplastic binder isremoved in a binder burnout step. The component is sintered to obtain afully consolidated and densified final structure. The FM compositecomponent is sintered in a pressureless, or essentially pressureless,furnace. The component is heated at temperatures and for a timeeffective for obtaining a predetermined degree of sintering. The finalresultant FM structure has desired properties such as strength, hardnessand fracture toughness.

[0025] Operating parameters of pressureless sintering are adjustedaccording to the material characteristics of the particular FM compositebeing sintered. These parameters are dictated in large part by themelting points of the constituents, their average particle sizes, aswell as presence of sintering aids. Gases such as N₂ and Ar can be usedin the sintering furnace to control the sintering environment. Anapplied overpressure of these gases (e.g., an overpressure of 6 psiapplied in the cold state or an overpressure of 30 psi in a hot state)may be used to improve sintering.

[0026] Sintering aids may be blended with one or more of the startingmaterials to enhance the sinterability of the FM composite. Sinteringaids are selected to be physically and chemically compatible with thestarting materials while possessing material properties such as lowermelting points, higher surface energy and/or higher atomic mobility. Inan example of liquid phase sintering, aluminum oxide and yttrium oxideare added to silicon nitride and at the sintering temperature of thesystem, a low viscosity melt is formed that effectively bonds thesilicon nitride grains together. Compositions that may be used assintering aids include aluminum oxide and yttrium oxide with siliconnitride, silicon carbide with zirconium carbide, zirconium metal withzirconium diboride, and hafnium hydride and carbon with hafnium carbide.The sintering aids are blended in amounts effective for enhancingconsolidation and densification of the FM composite during sintering toprovide a final FM composite structure with the desired FM properties.

[0027] In other embodiments, alternative methods of preparing FMfilaments and composite materials may be utilized. Alternativecompositions and methods, including those described in the co-pendingU.S. patent applications listed in Table 1, which are incorporated byreference herein in their entireties, are contemplated for use with thepresent invention. TABLE 1 ATTY FILING DOCKET TITLE INVENTORS DATE NO.ALIGNED Anthony C. Mulligan 12/04/2001 03248.0038 COMPOSITE Mark J.Rigali STRUCTURES Manish P. Sutaria FOR MITIGATION Dragan Popovich OFIMPACT DAMAGE AND RESISTANCE TO WEAR IN DYNAMIC ENVIRONMENTS METHODS ANDAnthony C. Mulligan 12/04/2001 03248.00040 APPARATUS FOR Mark J. RigaliPREPARATION Manish P. Sutaria OF THREE- Gregory J. Artz DIMENSIONALFelix H. Gafner BODIES K. Ranji Vaidayanathan COMPOSITE Mark J. Rigali12/04/2001 03248.00043 STRUCTURES Manish P. Sutaria FOR USE IN HIGH GregE. Hilmas TEMPERATURE Anthony C. Mulligan APPLICATIONS Marlene Platero-AllRunner Mark M. Opeka COMPOSITIONS Mark J. Rigali 12/04/200103248.00044 AND METHODS Manish P. Sutaria FOR PREPARING Felix GafnerMULTIPLE- Ron Cipriani COMPONENT Randy Egner COMPOSITE Randy C. CookMATERIALS MULTI- Anthony C. Mulligan 12/04/2001 03248.00045 FUNCTIONALJohn Halloran COMPOSITE Dragan Popovich STRUCTURES Mark J. Rigali ManishP. Sutaria K. Ranji Vaidyanathan Michael L. Fulcher Kenneth L. Knittel

EXAMPLES

[0028] The following examples are intended to illustrate the presentinvention and should not be construed as in any way limiting orrestricting the scope of the present invention.

Example 1

[0029] During hot-pressing of a Si₃N₄/BN FM composite, the BN cellboundary included glassy phases that were believed to result from themigration of sintering aids from the Si₃N₄ phase into the BN phase. Thismigration of glass appeared to aid the consolidation of the FMcomposite.

[0030] Sintering aids are blended directly with the BN phase to aid inthe consolidation process during pressureless sintering. Equivalentamounts of sintering aids as compared to the amount of glass present ina dense Si₃N₄/BN FM sample that was hot pressed are blended with the BNand thermoplastics composition during green processing. Sintering aidsfor use with Si₃N₄/BN FM composites are listed in Table 2. The sinteringaids listed in Table 2 are blended with BN, while standard sinteringaids (6 wt % Y₂O₃ and 4 wt % Al₂O₃) are blended with Si₃N₄. TABLE 2Weight Percentages System BN Al₂O₃ Y₂O₃ SiO₂ Borosilicate Si₃N₄ BN/YASGlass* 86.08 2.72 8.14 3.05 — — BN/YAS Glass* + 81.78 2.58 7.73 2.90 —5.00 Si₃N₄ BN/Borosilicate 75.00 — — — 25.00 — Glass

[0031] An amount of Si₃N₄ (about 5 wt. %) is added with the sinteringaids in the second system listed in Table 2, because Si₃N₄ is easilysintered and may enhance the sintering of BN in the FM system.

[0032] Si₃N₄/BN FM test bars with BN containing the glass sintering aidsare fabricated. To minimize porosity, the test bars are warmisostatically pressed prior to pressureless sintering. The test bars areplaced in a binder burnout furnace to remove polymer binders andpressureless sintered at 1750° C.

Example 2

[0033] Sintering experiments were conducted with ZrC/WRe FM compositesto establish consolidation conditions for more complex three-dimensionalcomponents such as bladed discs, nozzles and thrusters. The samples wereplaced in graphite crucibles and heated to temperature in a graphitefurnace in an argon atmosphere. The following sintering schedule wasused for the monolithic samples:

[0034] Room Temperature to 1200° C. at 25° C./min

[0035] 1200° C. to 2000° C. at 3.3° C./min

[0036] Hold at 2000° C. for 120 min

[0037] 2000° C. to 1000° C. at 10° C./min

[0038] 1000° C. to Room Temperature

[0039] The ZrC/WRe sample was sintered at 2000° C. for one hour. Theresults of the sintering experiments are shown in Table 3. TABLE 3Sintering Temperature Theoretical Density % Sample (°C.) Density (g/cc)Theoretical ZrC(5% HCS SiC) 2000 6.52 5.71 88 ZrC(10% HCS SiC) 2000 6.355.57 88 ZrC(15% HCS SiC) 2000 6.18 5.50 89 ZrC(20% HCS SiC) 2000 6.005.60 93 ZrC(15% PC SiC) 2000 5.83 5.44 88 ZrC(10% Zr) 2000 6.66 5.00 75ZrC(5% HCS SiC) 1950 6.52 5.13 79 ZrC(10% HCS SiC) 1950 6.35 5.01 79ZrC/WRe FM 2000 6.82 8.62 80 ZrC/WRe FM 2100 7.10 8.62 82

[0040] These experiments demonstrate that relatively high densities maybe achieved by sintering ZrC and ZrC/WRe FM composite samples. Theporosity of the samples was essentially closed as evaluated usingmicroscopic and scanning electron microscope (SEM) examinations. Thus,hot isostatic pressing of the samples produces parts at or very close tofull dense theoretical density.

Example 3

[0041] This example illustrates the preparation of a pressurelesssinterable multifilament zirconium carbide/boron nitride/zirconiumcarbide FM composite. Sinterable zirconium carbide powder with 15 volumepercent silicon carbide powder is blended with copolymers andplasticizer to form a fibrous monolith core material according to theformulation of Table 4. TABLE 4 Material Density (g/cc) Volume % Volume(cc) Weight (g) ZrC¹-15% SiC² 6.18 55.0% 24.75 152.96 EEA copolymer³0.93 32.0% 14.4   13.39 EAA copolymer⁴ 0.93  7.0%  3.15  2.92 MPEG-550⁵ 1.100  6.0% 2.7  2.97

[0042] A “Brabender” mixing machine (from C. W. Brabender of SouthHackensack, N.J.) is used to blend the above materials. The MPEG 550 isadded to adjust the blending torque of the composition to approximately200 kg-m².

[0043] In a separate process, boron nitride powder is blended withco-polymers and plasticizers to form the intermediate fibrous monolithboundary phase material according to the formulation shown in Table 5.TABLE 5 Material Density (g/cc) Volume % Volume (cc) Weight (g) BN⁶ 2.2750.0% 42.5  96.48 EEA copolymer⁷ 0.93 49.0% 22.05 20.51 MPEG-550⁵  1.100 1.0%  0.45  0.63

[0044] A “Brabender” mixing machine (from C. W. Brabender of SouthHackensack, N.J.) is used to blend the above materials. The MPEG 550 isadded to adjust the blending torque of the composition to approximately100 kg-m².

[0045] In a separate process, sinterable zirconium carbide powder with15 volume percent silicon carbide powder is blended with co-polymers andplasticizers to form the outermost layer of the fibrous monolithfilaments according to the formulation shown in Table 6. TABLE 6Material Density (g/cc) Volume % Volume (cc) Weight (g) ZrC⁹-15% SiC¹⁰6.18 50.0% 22.5 139.05  EEA copolymer¹¹ 0.93 40.0% 18.0 16.74 MPEG-550¹² 1.100 10.0%  4.5  4.95

[0046] A “Brabender” mixing machine (from C. W. Brabender of SouthHackensack, N.J.) is used to blend the above materials. The MPEG 550 isadded to adjust the blending torque of the composition to approximately100 kg-m².

Example 4

[0047] A multifilament zirconium carbide/boron nitride/zirconium carbidecontrolled geometry feed rod was assembled using the materials ofExample 3. A zirconium carbide feed rod was combined with a boronnitride shell. The zirconium carbide/boron nitride feed rod was loadedinto an extrusion cylinder and extruded at 105° C. A 2 millimeterdiameter zirconium carbide/boron nitride monofilament fiber was obtainedand collected on a motor controlled spooler. The zirconium carbide/boronnitride monofilament fiber was cut into 70 segments of about 5.5 inchesin length. The outermost zirconium carbide shell was loaded into amolding cylinder along with the 70 zirconium carbide/boron nitridemonofilament fiber segments. The assembly was pressed to form amultifilament feed rod of ZrC/BN filaments bundled within a ZrC shell.The feed rod was extruded to form a continuous length of 2 mm zirconiumcarbide/boron nitride/zirconium carbide multifilament fiber. Themultifilament fiber was then cut into 3 inch long segments and thenarranged into a 1 inch wide by 3 inch long by 0.25 inch thick couponsand molded to provide a green fibrous monolith ceramic structure. Fourgreen zirconium carbide/boron nitride/zirconium carbide fibrous monolithceramic coupons were prepared. Three of the four coupons were placed ingraphite crucibles and heated in a furnace in a nitrogen atmosphere toremove the binder in preparation for consolidation by pressurelesssintering. The fourth coupon was placed in a graphite hot press die andheated in a furnace in a nitrogen atmosphere to remove the binder inpreparation for consolidation by uniaxial hot pressing.

[0048] Two of the three zirconium carbide/boron nitride/zirconiumcarbide fibrous monolith ceramic coupons were consolidated bypressureless sintering in a nitrogen atmosphere using the followingschedule:

[0049] Room temperature to 2000° C. at 21.39° C./minute

[0050] Hold at 2000° C. for 120 minutes

[0051] 2000° C. to Room Temperature at 21.39° C./minute

[0052] The third zirconium carbide/boron nitride/zirconium carbidefibrous monolith ceramic coupon was consolidated by pressurelesssintering in a nitrogen atmosphere using the following schedule:

[0053] Room temperature to 1200° C. at 25° C./minute

[0054] 1200° C. to 2100° C. at 10° C./minute

[0055] Hold at 2100° C. for 60 minutes

[0056] 2100° C. to Room Temperature at 23.3° C./minute

[0057] The conditions of consolidation for the four zirconiumcarbide/boron nitride/zirconium carbide fibrous monolith ceramic samplesare presented in Table 7. Measured physical and mechanical properties ofthe four fibrous monolithic ceramic samples are provided in Table 8.TABLE 7 Molding Consolidation Consolidation Pressure ConsolidationConsolidation Temperature Pressure System (psi) Method Atmosphere (° C.)(ksi) ZrC/BN/ZrC  6,000 Pressureless Nitrogen 2000 0.014 SinteringZrC/BN/ZrC 12,000 Pressureless Nitrogen 2000 0.014 Sintering ZrC/BN/ZrC12,000 Pressureless Nitrogen 2100 0.014 Sintering ZrC/BN/ZrC  3,000 HotUni-axial Nitrogen 2200 4.0  Pressing

[0058] TABLE 8 Measured % Theoretical Consolidation Fracture StressDensity System Method (MPa) (cc) ZrC/BN/ZrC Pressureless  86 87.4  6000psi Sintering ZrC/BN/ZrC Pressureless 110 86.5 12000 psi SinteringZrC/BN/ZrC Pressureless   86. 87.8 12000 psi Sintering ZrC/BN/ZrC HotUni-axial 254 98.2  3,000 psi  Pressing

[0059] This experiment demonstrates that a ZrC/BN/ZrC fibrous monolithcomposite structure can be properly consolidated and densified bypressureless sintering.

[0060] Numerous modifications to the invention are possible to furtherimprove the methods for consolidation and densification. Thus,modifications and variations in the practice of the invention will beapparent to those skilled in the art upon consideration of the foregoingdetailed description of the invention. Although preferred embodimentshave been described above and illustrated in the accompanying drawings,there is no intent to limit the scope of the invention to these or otherparticular embodiments. Consequently, any such modifications andvariations are intended to be included within the scope of the followingclaims.

What is claimed is:
 1. A process for producing fibrous monolithcomponents comprising: combining a ceramic powder with a thermoplasticpolymer binder and a thermoplastic plasticizer to create a uniformlysuspended mixture, the uniformly suspended mixture comprising 50 to 62volume percent of the ceramic powder, 37 to 50 volume percent of thethermoplastic polymer binder, and 0 to 12 volume percent of thethermoplastic plasticizer; warm pressing the uniformly suspended mixtureinto a composite feed rod; extruding the composite feed rod with acomputer numerically controlled extruder to produce a fibrous monolithpreform; placing the preform in a binder burnout furnace to remove thethermoplastic polymer binder; placing the perform in a pressurelesssintering furnace to consolidate and densify the preform.
 2. The methodof claim 1 wherein the uniformly suspended mixture contains a sinteringaid.
 3. A process for consolidation and densification of fibrousmonolith components comprising: placing a preformed fibrous monolithcomposite in a sintering furnace, the sintering furnace containing aninert gas and having a pressure in the range of 1 to 30 Psi, applyingenergy to the fibrous monolith composite to achieve full density.
 4. Theprocess of claim 3 wherein the fibrous monolith composite comprisesSi₃N₄, BN, and a sintering aid.
 5. The process of claim 3 wherein thefibrous monolith composite comprises ZrC and WRe and is heated to atleast 2000 Celsius.
 6. A method for manufacture of an article comprisedof a fibrous monolithic material comprising the steps of: a) forming afibrous monolithic material in the form of a filament; b) compressingthe filament to consolidate the material and densify the material; c)forming the compressed filament into a preform of the article; and d)sintering the preform in an inert atmosphere at generally atmosphericpressure.